1. Field of the Invention
The present invention relates to a driving method of a CCD imaging device of a frame interline transfer type (hereinafter called an "FIT type"), and especially to a driving method of a solid-state imaging device for reducing the occurrence of a smear.
2. Description of the Related Art
Considerable improvement has been made with regard to a smear phenomenon which is a fault peculiar to the solid-state imaging device. Such improvement is remarkable in a so-called FIT-type CCD imaging device which is shown in FIG. 1. An imaging area 27 has a plurality of columns of photo-diodes 21 and a plurality of vertical CCD shift register 22, each of which is placed adjacent to each column of photo-diodes 21. A charge storage area 28 consists of a plurality of CCD charge transfer stages. A horizontal CCD shift register 25 shifts charges to an output portion 26, the charges having been transferred from the respective photo-diodes 21 through the vertical CCD shift registers 22 and the charge storage area 28. Charges that do not get shifted by the vertical CCD shift registers 22 to the storage area 28 end up in a charge drain portion 24.
An FIT-type CCD imaging device having a vertical overflow drain produces less smear than the amount found in an interline (IL) type CCD imaging device by increasing the frequency of frame transfer (FT). This reduces the smear to such a level as to enable the use of the imaging device as a broadcasting camera, although the use of the FIT-type device is accompanied by an enlarged chip area.
This FIT-type CCD imaging device enables the application of an electronic shutter mode. A conventional driving method will be described in regard to a normal mode and the shutter mode by using FIG. 1 and FIGS. 2(a) and 2(b).
In the normal mode, the solid-state imaging device shown in FIG. 1 is driven at a timing shown in FIG. 2(a). Specifically, a photoelectric conversion is made in a photo-diode 21 within a signal output period 12 in a one field period 11. A signal charge accumulated therein is read out to a vertical CCD shift register (hereinafter called a "V-CCD") 22 of an imaging area 27 by a read-out pulse 15. This charge is transferred subsequently to V-CCD 23 of the charge storage area 28 by a high-speed normal transfer pulse 16 and thereafter read out of a chip through a horizontal CCD shift register (hereinafter called "H-CCD") 25 and an output portion 26.
In the shutter mode, the signal charge in the photo-diode 21 is read out to V-CCD 22 by the read-out pulse 15 before a shutter accumulation period 14 is started, and this charge is swept out to the drain portion 24 by a high-speed sweep pulse 17 within a blanking period 13. Meanwhile, the photodiode 21 starts to accumulate the signal charge again with a commencement of the shutter period. The charge thus accumulated is read out of the chip by the same operation as in the normal mode. Since the time of the commencement of the shutter accumulation period 14 can be set arbitrarily in the shutter mode, a period wherein the photo-diode 21 accumulates the signal charge can be selected arbitrarily. According to this system, it is possible to control the amount of exposure and to prevent the blurring of an image of a subject moving at high speed.
The prior-art FIT-type CCD imaging device is faulty in that conspicuous deterioration in the smear occurs at the time of shuttering, though the smear at the time of an ordinary photosensing operation is reduced sufficiently.
The cause of this deterioration will be described with reference to FIGS. 3 and 4. The imaging device has a vertical overflow structure comprising the imaging area 27 (FIG. 1), the storage area 28, the H-CCD 25 and the drain portion 24 which are formed in a P-well region 41 at the surface portion of an N-type substrate 40, as shown in FIG. 3. The smear occurs when light 1 falling obliquely on a photo-diode 21 reflects off the surface thereof and further reflects off a photoshield 44 and enters V-CCD 22 of the imaging area 27 (see FIG. 1), or when a charge generated in the depth of the photodiode 21 (when light 2 falls thereon) is diffused into V-CCD 22 beyond a transfer gate 42, as shown in FIG. 3. The quantities of light 1 and 2 increase when a subject to be sensed is of high luminance.
In the FIT-type CCD imaging device, the smear is caused by charges leaking in at the time of a high-speed sweeping transfer high-speed normal transfer, or in a period between these transfers, the length of this period being the same both in the normal mode and in the shutter mode. Accordingly, the smear in the shutter mode is more pronounced in comparison with that in the normal mode as shown in FIG. 4. A signal amount in the shutter mode increases as indicated by line 55 while a signal amount in the normal mode increases as indicated by line 53 in this FIG. 4. A smear in the shutter mode, denoted by line 57, is equal to the smear in the normal mode in terms of the accumulation time and is higher in terms of the amount of smear per unit time. Therefore the smear in the shutter mode becomes larger than the smear in the normal mode, denoted by line 56.
In the case when a subject 61 of high luminance is present in the shutter mode, as shown in FIG. 5(a), a smear 62 occurs consequently above and below the subject 61. When the smear is swept out to a drain 24 (see FIG. 1) by a high-speed sweep pulse, a part of the smear caused by the subject 61 of high luminance is not swept out and remains in V-CCD 22. The part of V-CCD 22 wherein this smear is held receives the smear due to the subject 61 of high luminance, again during high-speed transfer. Therefore, the smear tends to occur in the lower-side image of the subject of high luminance even when it does not occur both above and below the subject 61. It happens to occur only in the lower part of the subject 61 of high luminance, as shown in FIG. 5(b).